Hot pressing is a technique for densifying green powder compacts or preforms using a combination of heat and pressure.
Hot pressing has principally been used to make parts having little volumetric detail. Such parts include plates, cylinders, simple straight tubes and the like. Many hot pressing applications have been directed to forming structural ceramic parts such as armor plates or wear parts. Such structural ceramic pieces do not usually have any surface or internal compositional features apart from, perhaps, a uniformly distributed composite phase.
A recent application of hot pressing technology has been in the area of ceramic substrates and electronic packaging for integrated circuit devices. U.S. Pat. No. 4,920,640 is a pioneer patent in this field. The contents of that patent are incorporated by reference in this application. The '640 patent discloses the use of hot pressing to form unmetallized and metallized single layer substrates and multilayer laminated substrates containing various configurations of metallization.
Another application of hot pressing was made in U.S. Pat. No. 4,753,694 to Herron et al. In Herron et al., a preformed laminate of metallization and glass-ceramic particles was densified to form a multilayered substrate at less than 1000.degree. C. In Herron et al., the preformed laminate was placed directly between two porous alumina platens which exerted pressure on the preform to prevent lateral shrinkage.
In a typical hot pressing process, the body to be sintered may contain ceramic particles to be sintered, a binder, and possibly a sintering aid. The binder is typically an organic binder which is removed by combustion or pyrolysis before sintering. As the ceramic sinters during hot pressing, intergranular phase material often forms. This material may result from the sintering aid additions and/or from impurities present in the starting ceramic particles. The presence of intergranular phase is usually desirable as it assists the sintering process.
For sintering ceramic particles, hot pressing conditions typically involve temperatures in excess of 1000.degree. C., usually in excess of 1200.degree. C. These temperatures are combined with varying degrees of pressure to enhance and/or accelerate the sintering process.
While intergranular phases are generally desirable to assist sintering, the behavior of some of these phases can be detrimental to the sintered product. Certain liquid phases tend to wet the ceramic grain boundaries. If those phases remain after sintering, the high temperature mechanical properties of the sintered product may be degraded. For example, such wetting phases may promote grain sliding or other phenomena which contribute to high temperature creep.
Additionally, since the wetting phase represents a microstructural inhomogeneity between grains, the wetting phase may affect other properties as well (e.g., it may lower the effective thermal conductivity of the body).
Thus, for some materials systems, it may be desirable to hot press under conditions so liquid phase sintering occurs, but the liquid phase is eventually removed by evaporation from the grain boundary surface to improve material properties. Such methods of hot pressing and liquid phase removal present additional problems. Namely, since hot press tooling encloses the ceramic part, uniform removal of the liquid phase from the part is often difficult or impossible to achieve. Non-uniform liquid phase removal can cause non-uniformity of properties in the sintered product.
Thus, there is a need for an improved hot pressing technique which allows use of intergranular phase material during sintering while avoiding the problems caused by certain intergranular phases.